Researchers from Arizona State University (ASU) in the US seem to have hit upon the key factor in urban pollution which has an effect on global warming.
According to Peter Crozier and James Anderson from ASU, so-called brown carbons, a nanoscale atmospheric aerosol species, are largely being ignored in broad-ranging climate computer models.
Studies of the greenhouse effect that contribute directly to climate change have focused on carbon dioxide and other greenhouse gases.
But there are other components in the atmosphere that can contribute to warming - or cooling - including carbonaceous and sulfate particles from combustion of fossil fuels and biomass, salts from oceans and dust from deserts.
Brown carbons from combustion processes are the least understood of these aerosol components.
The parameter typically used to measure degrees of warming is radiative forcing, which is the difference in the incoming energy from sunlight and outgoing energy from heat and reflected sunlight.
The variety of gasses and aerosols that compose the atmosphere will, under different conditions, lead to warming (positive radiative forcing) or cooling (negative radiative forcing).
According to the ASU researchers, the effect of brown carbon is complex because it both cools the Earth's surface and warms the atmosphere.
"Because of the large uncertainty we have in the radiative forcing of aerosols, there is a corresponding large uncertainty in the degree of radiative forcing overall," said Crozier. "This introduces a large uncertainty in the degree of warming predicted by climate change models," he added.
A key to understanding the situation is the light-scattering and light-absorbing properties - called optical properties - of aerosols.
Crozier and Anderson are trying to directly measure the light-absorbing properties of carbonaceous aerosols, which are abundant in the atmosphere.
"If we know the optical properties and distribution of all the aerosols over the entire atmosphere, then we can produce climate change models that provide more accurate prediction," said Anderson.
Most of the techniques used to measure optical properties of aerosols involve shining a laser through columns of air.
Anderson and Crozier have instead used a novel technique based on a specialized type of electron microscope.
This technique - monochromated electron energy-loss spectroscopy - can be used to directly determine the optical properties of individual brown carbon nanoparticles over the entire visible light spectrum as well as over the ultraviolet and infrared areas of the spectrum.
"We have used this approach to determine the complete optical properties of individual brown carbon nanoparticles sampled from above the Yellow Sea during a large international climate change experiment," said Crozier.
"This is the first time anyone has determined the complete optical properties of single nanoparticles from the atmosphere," said Anderson.